U.S. patent number 4,766,498 [Application Number 07/018,301] was granted by the patent office on 1988-08-23 for image projection system.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Johannes H. Spruit.
United States Patent |
4,766,498 |
Spruit |
August 23, 1988 |
Image projection system
Abstract
An image projection system has at least one display tube
provided with an interference filter for increasing the amount of
light which is emitted within a small solid angle. Each display
tube is combined with a projection lens system having a first lens
element of positive power with an aspherical surface at the image
end and a convex aspherical surface at the object end, which
surface has a center of curvature in the image end half. A second
lens element of negative power has a concave surface facing the
image end, which is toward the first element, and an opposed
surface which conforms to the display window of the display tube. A
third lens element which is bi-convex may be located between first
and second lens elements. The pupil of the lens system is located
in the image end half of the first element. The brightness of the
picture on a projection screen is enhanced while the brightness
variation and the color shading of this picture are reduced.
Inventors: |
Spruit; Johannes H. (Eindhoven,
NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
19848874 |
Appl.
No.: |
07/018,301 |
Filed: |
February 24, 1987 |
Foreign Application Priority Data
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Nov 24, 1986 [NL] |
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8602975 |
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Current U.S.
Class: |
348/781;
348/E9.025 |
Current CPC
Class: |
G02B
13/18 (20130101); G02B 5/285 (20130101); H04N
9/31 (20130101); G02B 13/16 (20130101) |
Current International
Class: |
G02B
13/18 (20060101); G02B 13/16 (20060101); G02B
5/28 (20060101); H04N 9/31 (20060101); H04N
005/74 () |
Field of
Search: |
;358/60,64,231,237,238,250 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
170320 |
|
Feb 1986 |
|
EP |
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2091898 |
|
Aug 1982 |
|
GB |
|
Primary Examiner: Chin; Tommy P.
Attorney, Agent or Firm: Faller; F. Brice
Claims
I claim:
1. An image projection system comprising at least one display tube
and a multi-element projection lens system for imaging a display
tube picture on a projection screen, said lens system having an
image end toward said screen, an object end toward said display
tube, and a radiation path extending therebetween, said display
tube comprising a display screen in an evacuated envelope, the
display screen being arranged on the inside of a display window in
the wall of the envelope and being provided with a luminescent
material, a multi-layer interference filter being arranged between
said luminescent material and the display window, characterized in
that a first lens element of the projection lens system, viewed
from the image end, is of positive power and has a first refractive
aspherical surface facing the image end and a second refractive
surface facing the object end, which surface is convex and whose
centre of curvature is located in the half of said first lens
element toward said image end, in that the projection lens system
further comprises a second lens element of negative power having a
concave surface facing the image end and an opposed surface having
a curvature adapted to that of the display window and in that the
projection lens system has an entrance pupil located in the image
end half of the first lens element.
2. An image projection system as claimed in claim 1, wherein the
first refractive aspherical surface of the first lens element,
viewed from the image end, is convex in a zone around the optical
axis and is concave outside said zone.
3. An image projection system as claimed in claim 2, wherein the
second refractive surface of the first lens element is
aspherical.
4. An image projection system as claimed in claim 1 further
comprising a third bi-convex lens element supplying a part of the
overall optical power of the projection lens system and arranged
between the first and the second lens elements.
5. An image projection system as claimed in claim 4, wherein the
optical power of the third lens element is approximately equal to
that of the first lens element.
6. An image projection system as claimed in claim 4 wherein the
surface of the third lens element facing the image end is
aspherical.
7. An image projection system as claimed in claim 4 wherein the
surface of the first and third lens elements facing the object end
are aspherical.
8. An image projection system as in claim 1 wherein the first lens
element has a third surface which is reflective and is located in
the radiation path between the first and the second refractive
surfaces.
9. An image projection system as in claim 1 wherein the display
window of the display tube, viewed from the image end, is concave
and has an angle of curvature .phi. wherein .phi. is the angle
between the normal in the centre of the display window and a line
perpendicular to the edge of the display window, which angle .phi.
has a value in the range between 5.degree. and 25.degree..
Description
BACKGROUND OF THE INVENTION
The invention relates to an image projection system comprising at
least one display tube and a multi-element projection lens system
for imaging a display tube picture on a projection screen. The
display tube has a display screen in an evacuated envelope, the
display screen being arranged on the inside of a display window in
the wall of the envelope and being provided with a luminescent
material, a multi-layer interference filter being arranged between
said material and the display window. The invention also relates to
a projection device provided with such an image projection
system.
As is described in European Patent Application No. 0170230, use of
an interference filter consisting of a large number of layers, for
example fourteen to thirty, may considerably improve the display
tube properties regarding the intensity distribution within and the
monochromaticity of the beam emitted by the tube. The interference
filter passes light of the desired wavelength in the direction
perpencidulat to the display screen and in directions which extend
at angles of, for example 30.degree. at a maximum to the normal on
this screen. These directions may be designated the forward
directions. Light emerging from the luminescent layer at larger
angles to the normal, up to approximately 90.degree., is reflected
to this layer by the interference filter. This light is dispersed
within the luminescent layer and is then emitted in the forward
directions so that the brightness in these directions is enhanced.
Such a tube can be used advantageously in a television projection
system.
The light of wavelengths shorter than the desired wavelength is
passed by the filter also at larger angles, so that for these
wavelengths the gain in intensity in the forward directions is
smaller than that for the desired wavelength. For wavelengths which
are larger than the desired wavelength, the maximum angle at which
the light passes through the filter is smaller than that for the
light of the desired wavelength and the filter can even completely
block the larger wavelengths in the forward directions. The
interference filter therefore not only ensures that the emitted
light is concentrated into a smaller solid angle thus enabling the
amount of light received by the projection lens system to increase
but it has also a wavelength-selective effect so that the risk of
chromatic aberrations in the projection lens system is reduced and
the projected image may have a better contrast.
It has been found that satisfactory results can be achieved when
using display tubes provided with an interference filter in
projection systems if such tubes are combined with projection lens
systems having relatively larger focal lengths, for exampler of the
order of 130 mm. For such a projection lens system the entrance
pupil is at a relatively large distance from the display screen of
the tube. The entrance pupil is defined as the image of the
physical boundary or "stop" in the lens system, which image is
formed by the lens elements which are present at the object end of
this boundary. The entrance pupil is located at the position in
which the chief ray of a beam obliquely incident on the lens system
intersects the optical axis of this system. If the entrance pupil
is located at a relatively large distance from the display screen
of the tube, the field angle of the projection lens system is
relatively small so that only the light emitted by the display tube
in the forward directions is received by the lens system.
The object or image field angle as used herein means the angle
between the optical axis of the lens system and the chief ray of a
beam originating from the corner of the object and a beam being
directed onto the corner of the image, respectively, which beam
passes through the lens system with a still acceptable vignetting.
The corner of the object is the end of a diagonal of the image
written on the display window of the picture display tube. The
corner of the image is the end of the diagonal of the picture
formed on the projection screen.
In connection with the increasing demand for projection systems,
notably colour television projection systems which can be
accommodated in a cabinet having a smaller volume there is an
increasing need of a projection lens system having a relatively
small focal length, for example smaller than 80 mm because then the
overall required optical path length from the display tube to the
projection screen is also relatively small. Decreasing the focal
length of a projection lens system in a projection system will
generally result in that the entrance pupil of the lens system will
be located closer to the display screen of the display tube. This
means that the object field angle becomes larger and that notably
in the corner of the display window light which is emitted in the
non-forward directions is received by the projection lens system
and thus reaches the projection screen. The picture on the
projection screen observed by the viewers then exhibits a variation
in brightness from the centre of the picture to the edge thereof.
This is a result of the fact that the interference filter in the
display tube reduces the amount of light in the non-forward
directions. This first variation in brightness is still augmented
by a second type of brightness variation which is caused by the
larger angle at which the chief ray of a beam originating from the
corner of the display tube passes through the projection lens
system and by vignetting at the lens elements.
A second result of increasing the field angle is that a colour
shading occurs on the projection screen due to the colour-selective
effect of the interference filter in the display tube. In this case
the colour shading implies that there is a shift to shorter
wavelengths, thus to blue, from the centre of the projection screen
to the edge.
In order to eliminate the problems of brightness variation and
colour shading an interference filter could be arranged in the
display tube to pass light of the desired wavelengths at larger
angles. However, there will be a considerably smaller gain in
brightness while colour shading will still occur.
SUMMARY OF THE INVENTION
The present invention has for its object to eliminate the drawbacks
while maintaining the advantages of the use of an interference
filter in the display tube and even making better use thereof.
A first lens elements of the projection lens system, viewed from
the image end, is constituted by a positive lens element having a
first refractive aspherical surface facing the image end and a
second refractive surface facing the object end, which second
surface is convex and has a centre of curvature located in the
image end half of this lens element projection lens system includes
a second lens element of negative power and having a concave
surface facing the image end and an opposed surface having a
curvature adapted to that of the display window. The entrance pupil
of the projection lens system is located in the image end half of
the first lens element.
The projection lens system has a relatively small focal length, for
example 78 mm or 90 mm whilst in addition the field angle as
defined in the foregoing is small. In principle the first lens
element can ensure that, with the field angle at the image end
remaining equal, the field angle at the object end is reduced by a
factor which is equal to the refractive index of the material of
this element. Viewed in the reverse radiation path, hence from the
projection screen to the display tube, the first lens element may
form an intermediate image which exhibits substantially no coma and
astigmatism.
To reduce the spherical aberration in the intermediate image, the
first refractive surface of the first lens element is
aspherical.
The aspherical surface of the first lens element, viewed from the
image end, is preferably convex in a zone around the optical axis
and concave outside said zone.
At the reduced field angle the further imaging in the reverse
radiation path, with the beam already rendered converging by the
first lens element, can in principle be realized by means of only
the second, concave lens element. This element may be referred to
as field curvature correction lens element or field flattener. The
concave surface of this lens element may be aspherical.
In a projection lens system having only a first and a second lens
element the second refractive surface of the first lens element is
preferably aspherical.
The remaining aberration in the image formed can then be
reduced.
A third bi-convex lens element supplying a part of the overall
optical power of the projection lens system may be arranged between
the first and the second lens element. The optical power of the
third lens element is preferably approximately equal to that of the
first lens element.
By addition of the third lens element the numerical aperture can be
increased and aberrations can be reduced, whilst the focal length
remains equal. In this embodiment of the projection lens system the
physical pupil is located closer to the centre of the first lens
element in order to prevent vignetting but this pupil is still
located in the first portion of this lens element.
In a first embodiment of the projection lens system comprising
three lens elements the surface of the third lens element facing
the image end may be aspherical. This contributes to a reduction in
aberrations of oblique beams in the projection system.
In a second embodiment of the projection lens system comprising
three lens elements the surfaces of the first and third lens
elements facing the object end are aspherical. This projection lens
system thus has four refractive surfaces.
In a most preferred embodiment of the projection lens system, the
first lens element has a third surface which is reflective and is
located in the radiation path between the first and the second
refractive lens surface.
This first lens element may be designated as a folded lens element.
The optical path length in this lens element is equal to that in a
lens element which is twice as thick, but the required quantity of
optically high-grade lens material and consequently the weight
amount to only half that of the corresponding thick lens element.
In addition the reflecting surface changes the direction of the
imaging beam in such a way that the radiation path of this beam can
be folded in an optimum manner while maintaining the overall
optical path length so that the projection system can be
accommodated in a cabinet of relatively small volume.
It is to be noted that U.S. Pat. No. 4,526,442 describes a
projection lens system having a reflecting surface. However, in the
known system the reflector is arranged between the first and the
second lens element and this reflector does not form part of the
first lens element. Moreover, the known projection lens system has
a relatively large focal length.
Optimum use is made of the inventive idea in an image projection
system which is further characterized in that the display window of
the display tube, viewed from the image end, is concave and has an
angle of curvature .phi. wherein .phi. is the angle between the
normal in the centre of the display window and a line perpendicular
to the edge of the display window, which angle .phi. has a value in
the range between 5.degree. and 25.degree.. By using a concave
display window the design of the projection lens system can be
simplified. The concave shape of the display window aan increased
concentration of the radiation of the display tube in the direction
of the entrance pupil of the projection lens system and hence a
better light distribution across the projection screen is obtained.
By combination of a concave display window and an interference
filter in the display tube an improvement of the projected image is
obtained which is greater than the sum of the improvements obtained
by the separate use of the concave display window and the
interference filter, respectively.
The concave display window need not be spherical but may have a
different shape, for example an aspherical shape.
It is to be noted that it is known per se from British Patent
Application No. 2,091,898 to use a concave display window in order
to be able to simplify the design of the projection lens system.
However, this Patent Application does not reveal that the light
output, notably in the corner of the display window, can be
considerably increased by combination of a concave display window
and an interference filter between this display window and the
luminescent material.
The invention may be used in a projection system with one display
tube for forming, for example a black-green image on the projection
screen.
The greatest advantage of the inventive idea can be achieved when
using it in a colour picture projection device. An embodiment of
such a device, which has three separate colour channels which
converge on a common projection screen and which are each provided
with a display tube and a projection lens system, is characterized
in that at least one of the colour channels is provided with an
image projection system as described above.
The use of the inventive idea is most desirable in the green
channel, because the electron current in the display tube must be
large for this channel in order to obtain the desired light
intensity. However, the combination of the interference filter and
the projection lens system according to the invention may also be
arranged in the red channel and possibly in the blue channel.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic perspective view of a color picture
projection device provided with three image projection systems,
FIG. 2 is a cut away perspective of a display tube for image
projection,
FIGS. 3 and 4 are partial cross-sections of display tubes having
straight and curved display windows, respectively, and an
interference filter,
FIG. 5 diagrammatically shows the composition of the interference
filter,
FIG. 6 shows different spectra of radiation emitted by a green
phosphor display tube,
FIG. 7 shows the basic shape of the first lens element,
FIG. 8 shows a first embodiment of the projection lens system,
FIG. 9 shows a second embodiment of this projection lens
system,
FIG. 10 shows the preferred embodiment of this system,
FIG. 11 shows the magnitude of the angle of acceptance as a
function of the focal length of the projection lens system in
projection systems having a conventional and the novel projection
lens system, respectively.
FIGS. 12 and 13 show the difference in brightness variation across
the projection screen when using a conventional and the novel
projection lens system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 includes a colour television receiver 1. An input of this
receiver coupled to an antenna 2 receives a colour television
signal which is split up into a red, a green and a blue signal.
These signals are applied to three separate display tubes 3, 4 and
5 whose display windows show a red, a green and a blue picture.
These pictures are projected on a projection screen 10 by the
projection lens systems 6, 7 and 8 shown diagrammatically and
associated with the separate tubes. For the sake of clarity only
the chief rays of beams imaging a point of a display window on a
point of the projection screen are shown. The radiation emitted by
each display tube and passing through the associated projection
lens system covers the entire projection screen 10. A mirror 9
reflecting the beams emerging from the projection lens systems to
the projection screen is arranged between the projection lens
systems and the projection screen. This mirror folds the radiation
path so that the projection system can be accommodated in a cabinet
having a relatively small depth without having to shorten the
radiation path.
The three single-colour pictures must be super-imposed on the
projection screen. To this end the aligned display tubes are
slightly bent towards each other, that is to say the normals on the
screens of the tubes 3 and 5 extend at a small angle to the normal
on the screen of the tube 4.
In the projection screen 10 the radiation of the three beams is
dispersed over a relatively large angle in the Y-direction, that is
to say in the horizontal direction for the viewer W, while in the
Z-direction, the vertical direction for the viewer W, the radiation
is dispersed through a smaller angle. The viewer W sees a picture
which is the superimposition of the magnified pictures of the
display tubes.
FIG. 2 shows one of the display tubes 4 in a perspective view in
which a part of the glass envelope 20 is broken away. The glass
envelope constitutes a display window 21 on the front side and
comprises a cone 22 and a neck portion 23. A luminescent screen not
shown in FIG. 2 is provided on the inside of the display window and
an electron gun 24 is arranged in the neck of the tube. A system 25
of deflection coils around the neck of the tube ensures that the
electron beam scans the luminescent screen in two mutually
perpendicular directions Y and Z. The electrical connections to the
receiver are established by means of connection pins 27 in the base
26.
FIGS. 3 and 4 show cross-sections of a flat display window 21 and a
display screen 27 and of a concave display window 21' and a display
screen 27', respectively. The insets in these Figures show the
display windows 21 and 21', the multilayer interference filters 28
and 28' and the display screen 27 and 27', respectively. The
display screen consists of a layer of luminescent material, a
phosphor 29 and 29' respectively and a thin aluminium film 30 and
30', respectively, the so-called "aluminium backing".
The curved display window of FIG. 4 has an angle of curvature .phi.
and is preferably spherical with a radius of curvature .rho.. The
display window may be alternatively aspherical, for example.
The interference filter is diagrammatically shown in FIG. 5. This
filter comprises a large number of layers, for example, twenty
alternately having high (H) and low (L) refractive indices. These
layers are approximately 0.25.lambda..sub.f thick at an average in
which .lambda..sub.f =p..lambda. where .lambda. is the desired
central wavelength which is selected from the spectrum emitted by
the luminescent material and where p is between 1.18 and 1.32. The
layers L comprise, for example SiO.sub.2 having a refractive index
n=1.47 or MgF.sub.2 having a refractive index n=1.38. The layers H
may comprise TiO.sub.2 with n=2.35, Ta.sub.2 O.sub.5 with n=2.00 or
Nb.sub.2 O.sub.5 with n=2.15. The last layer 32 on the side of the
display screen 27 is covered with, for example a
0.125.lambda..sub.f thick separation layer 31 having a low
refractive index.
For further particulars of the interference filter and embodiments
thereof reference is made to the European Patent Application No.
0,170,320 in the name of the Applicant. As is described in this
European Patent Application, the provision of the interference
filter has the advantages that
the quantity of light which is emitted by the display tube within a
given solid angle which may be, for example 25.degree. but also
40.degree. can be, for example 40% to 50% larger than in the case
of a display tube without such a filter and
the intensity of the desired wavelength is increased at the expense
of that of the unwanted wavelengths, so that the beam emitted by
the tube becomes more monochromatic.
For a projection system as a whole these advantages result in:
a greater brightness on the projection screen,
an improvement of the colour in the picture on the projection
screen, particularly noticeable in the green colour in the case of
a colour picture projector,
less chromatic aberration in the projection lens system,
particularly noticeable in the green and blue channels in the case
of colour picture projection and
a better contrast in the picture on the projection screen.
For the purpose of illustration FIG. 6 shows by way of example the
spectra emitted by the green display tube under different
circumstances. The luminescent material of this tube is a phosphor
of the composition YAg:Tb. The curve 6A shows the spectrum which is
emitted if no interference filter is present in the display tube.
The curves 6B to 6E show the spectra which in the presence of an
interference filter are emitted in the directions which extend at
angles of 0.degree., 15.degree., 30.degree. and 45.degree.
respectively, to the normal on the display window. A comparison of
the curves 6A and 6B shows that the interference filter enhances
the brightness of the desired spectral range in the direction
perpendicular to the display window, whilst the brightness of the
spectral line at 545 nm is enhanced in comparison with the other,
blue spectral line at wavelengths of less than 500 nm. These
advantages are maintained for directions which extend at angles up
to a maximum of 30.degree. to 35.degree. to the normal on the
display window as is shown in FIGS. 6c and 6d. Curves analogous to
those of FIG. 6, but of course with different spectral lines apply
to the blue and the red display tubes. For directions at angles of
more than 30.degree. to 35.degree. to the normal the brightness of
the green spectral line decreases in FIG. 6, whereas that of the
blue spectral line remains approximately equal as is shown in FIG.
6 by the curve 6E which applies to a direction at 45.degree. to the
normal on the display window.
In order to prevent that colour shading occurs across the
projection screen due to the relatively larger contribution of the
blue spectral line with respect to the green spectral line and to
prevent the brightness decreasing to a considerable extent from the
centre of the projection screen to the edge thereof due to the
decrease in brightness of the green spectral line, it is ensured
according to the invention that light which is emitted at angles of
more than 30.degree. to 35.degree. to the normal on the display
window is not received by the projection lens system. A projection
lens system is provided which has a small field angle at the object
and, whilst the focal length is also small. Referring to FIG. 7, a
thick plano-convex lens L.sub.1 ' has convex lens surface S.sub.3 '
with a center of curvature M located on the flat surface S.sub.1 '.
Although in the projection system the rays extend from right to
left, the ray path in FIG. 7 is shown from left to right. This
inversion is permitted in this type of optical systems and is used
here because it simplified the insight into the operation of the
system. The beam b.sub.1 shown in FIG. 7 is an imaginary beam which
originates from the centre of the projection screen. Due to the
relatively large distance between the lens element L.sub.1 ' and
the projection screen the beam b.sub.1 is only slightly-diverging.
This beam whose chief ray coincides with the optical axis 00' of
the lens element L.sub.1 ' is converged in the point F.sub.1 on the
optical axis. The reference symbol b.sub.2 denotes a likewise
imaginary, slightly diverging beam originating from the edge of the
projection screen and passes through the lens element L.sub.1 '
with a still acceptable vignetting. The chief ray of this beam
intersects the optical axis in the point M. For this lens element
the pupil PP', the entrance pupil viewed from the projection
screen, and the exit pupil viewed from the image source, is
therefore located at the area of the first refractive surface
S.sub.1 ' of the lens element L.sub.1 '. The chief ray of the beam
b.sub.2 is perpendicularly incident on the convex surface S.sub.3 '
and passes this surface without refraction. The peripheral rays of
the beam b.sub.2 are refracted towards the chief ray by the surface
S.sub.3 ' so that the beam b.sub.2 is focussed in the point
F.sub.2.
By using a lens element L.sub.1 ' in accordance with FIG. 7 in a
projection system, it is achieved that the field angle at which the
edge of the projection screen is viewed by the elements of the
projection lens system located between the image source and the
lens element is reduced. The field angle is the angle .alpha. at
which the chief ray of the beam b.sub.2 incident on the surface
S.sub.1 ' intersects the optical axis. After refraction by this
surface this chief ray extends at an angle .beta. to the optical
axis. The field angle reduction is given by ##EQU1## wherein
n.sub.2 is the refractive index of the lens material and n.sub.1 is
that of the surrounding medium. If this medium is air for which
n.sub.1 =1 the field angle is reduced by a factor which is
approximately equal to n.sub.2.
For this reduced field angle and the convergence of the beams
brought about by the lens element L.sub.1 ' the further imaging may
be realized with only several simple additional lens elements. In
certain uses it would be sufficient to use only a concave lens
L.sub.3 as shown in FIG. 8. This lens element is the field
curvature correction lens or "Field Flattener". The concave surface
of the lens element L.sub.3 is denoted by S.sub.6 in FIG. 8. The
curvature of the second surface S.sub.7 of the lens element L.sub.3
is adapted to that of the display window FP of the display tube not
further shown. In the embodiment shown in FIG. 8 this display
window and thus also the second surface S.sub.7 is concave as
viewed from the image end.
A plate-shaped holder CP may be arranged between the display window
FP and the surface S.sub.7 of the lens element L.sub.3. A coolant
such as water and glycol flows through this holder since without
cooling the temperature of the luminescent material which is
provided in the display screen could increase considerably so that
the brightness of the tube could decrease.
The lens L.sub.1 ' may form an intermediate image which is
diagrammatically shown in FIG. 7 by the points F.sub.1 and F.sub.2,
which image has substantially no coma and astigmatism but is still
beset with spherical aberration. This aberration may be reduced by
giving the first surface S.sub.1 of the lens element L.sub.1 ' an
aspherical shape. In order to realize a further reduction of
aberrations, the second surface S.sub.3 " of the lens element
L.sub.1 ' may also be aspherical. Since also the concave surface
S.sub.6 of the lens element L.sub.3 is aspherical, the projection
lens system of FIG. 8 totals three aspherical surfaces. This system
may have a focal length of 78 mm and a numerical aperture at the
object end of 0.30 to 0.35.
A considerable increase of the numerical aperture and a reduction
of aberrations at the same focal length can be obtained by adding a
third biconvex lens element. Such a projection lens system, which
is not only extremely suitable for projecting present-day
television images but also for projecting future high-definition
television images, is shown in FIG. 9. The optical power of the
projection lens system is now distributed over the first thick lens
element L.sub.1 ' and the third lens element L.sub.2. These lens
elements preferably have approximately the same optical powers. To
this end the lens element L.sub.1 ' is slightly adapted but its
basic shape has been maintained. The pupil PP' of the system is
shifted slightly to the object end but is still located fairly
close to the first refractive surface S.sub.1 so that the field
angle reduction is comparable to that which is obtained in the
systems according to FIGS. 7 and 8. If the surfaces S.sub.1,
S.sub.4 and S.sub.6 are aspherical surfaces, a numerical aperture
of more than 0.40 at a focal length of 78 mm can be realized.
The lens element L.sub.1 ' in FIGS. 7 and 8 is a thick glass lens
element and is thus relatively costly and heavy. As already shown
in FIG. 9, the projection lens system is designed in such a way
that this lens element becomes even slightly thicker so that a
diagonal plane d can be provided through the lens element. By
leaving out the lens material under this plane, rendering the
surface reflective at the location of this plane and providing the
surface S.sub.1 on the upper side of the lens element, a folded
lens element (L.sub.1 in FIG. 10) is obtained which exhibits the
same behaviour as the lens element L.sub.1 ' of FIG. 9. The folded
lens element L.sub.1 has the advantage that it requires half the
quantity of optically high-grade lens material needed for the lens
element L.sub.1 ' of FIG. 9 so that the folded lens element is
considerably lighter and less costly. Besides the surface S.sub.2
reflects the projection beam in a direction which is optimum for
further folding the radiation path in the projection system.
The surface S.sub.2 of the lens element L.sub.1 may be rendered
reflective by providing a silver layer, for example by means of
vapour deposition.
Also in the projection lens system of FIG. 8, having only two lens
elements, the first lens element may be replaced by a folded lens
element. This projection lens system preferably comprises three
aspherical surfaces S.sub.1 ', S.sub.3 ' and S.sub.6.
In the embodiments of FIG. 10 the surfaces S.sub.1, S.sub.4 and
S.sub.6 may be aspherical.
The optical behaviour of the lens elements L.sub.1 and L.sub.2,
which determine the optical power and the focal length of the
projection lens system, must be independent of variations in
temperature or humidity of the surrounding medium. On the other
hand these lens elements must have aspherical surfaces which are
difficult to realize in glass. Therefore these lens elements
preferably consist of glass substrates L.sub.1,s and L.sub.2,s
which on their aspherical sides carry thin layers L.sub.1,a and
L.sub.2,a of a transparent synthetic material with aspherical outer
profiles S.sub.1 and S.sub.4, respectively. S.sub.1,a and S.sub.4,a
are the inner surfaces of the thin layers L.sub.1,a and L.sub.2,a
respectively. Since the layers are thin, a variation of the
refractive index or of the shape of these layers as a result of
variations in the ambient parameters has only a slight effect on
the behaviour of the lens elements as a whole.
The projection lens system according to FIG. 10 may have four
instead of three aspherical surfaces, namely the surfaces S.sub.1,
S.sub.3, S.sub.5 and S.sub.6. A glass having a lower refractive
index can then be used for the lens elements L.sub.1 and
L.sub.2.
The aspherical layers may be provided on the lens substrates by
means of a so-called replica process. In this process use is made
of moulds having inner profiles which are the reverse of the
desired outer profiles of the layers to be formed. A transparent
synthetic material brought to a sufficiently soft condition, for
example, a synthetic material which can be polymerized by
ultraviolet radiation is provided on a lens substrate whereafter a
mould is pressed into it. Subsequently the synthetic material is
cured, for example by irradiation by ultraviolet light and the
mould is removed and the lens becomes available without any further
processing operations being required.
The correcting lens element L.sub.3 may entirely consist of a
synthetic material, for example, polymethylmethacrylate (PMMA) or
polycarbonate (PC). The aspherical profile on the surface S.sub.6
may already be provided during moulding of the lens element by
making use of a mould having an aspherical profile. It is
alternatively possible to provide the aspherical profile after
forming the lens element L.sub.3 by means of the replica
process.
The aspherical surfaces, for example, S.sub.1, S.sub.4 and S.sub.6
of FIG. 10 may be characterized by ##EQU2## wherein Y is the
distance between a point on the aspherical surface and the optical
axis of the lens element and Z is the distance between the
projection of this point on the optical axis and the point of
intersection of the optical axis with the aspherical surface.
The following values apply, from the image end, to the axial
surface curvatures C, the axial distances di between these surfaces
and the refractive indices n for an embodiment of the projection
lens system according to FIG. 10 in which the lens element L.sub.1
consists of glass of the type number SF.sub.2 from Messrs. Schott
and the lens elements L.sub.2 and L.sub.3 consist of polycarbonate,
whose focal length is 78 mm and the numerical aperture is
0.425.
______________________________________ C(mm.sup.-1) di(mm) n
______________________________________ S.sub.1 0.005679 L.sub.1
84.00 1.654 S.sub.3 -0.009203 0.100 S.sub.4 0.007552 L.sub.2 16.00
1.573 S.sub.5 -0.003645 45.48 S.sub.6 -0.015772 L.sub.3 5.00 1.573
S.sub.7 -0.028571 ______________________________________
whilst the aspherical coefficients a.sub.2i of the surfaces
S.sub.1, S.sub.4 and S.sub.6 are equal to
______________________________________ S.sub.1 a.sub.2 = 0.283935
.times. 10.sup.-2 a.sub.4 = -0.390136 .times. 10.sup.-6 a.sub.6 =
-0.750233 .times. 10.sup.-9 a.sub.8 = 0.839881 .times. 10.sup.-12
a.sub.10 = -0.564121 .times. 10.sup.-15 a.sub.12 = 0.142924 .times.
10.sup.-18 S.sub.4 a.sub.2 = 0.377615 .times. 10.sup.-2 a.sub.4 =
0.301339 .times. 10.sup.-6 a.sub.6 = 0.243433 .times. 10.sup.-9
a.sub.8 = -0.190848 .times. 10.sup.-12 a.sub.10 = 0.873343 .times.
10.sup.-16 a.sub.12 = -0.138625 .times. 10.sup.-19 S.sub.6 a.sub.2
= -0.788596 .times. 10.sup.-2 a.sub.4 = -0.486486 .times. 10.sup.-5
a.sub.6 = 0.201054 .times. 10.sup.-8 a.sub.8 = -0.821263 .times.
10.sup.-12 a.sub.10 = 0.192444 .times. 10.sup.-15 a.sub.12 =
-0.140404 .times. 10.sup.-19
______________________________________
For an embodiment of the projection lens system according to FIG.
10 in which the lens element L.sub.1 consists of glass of the type
number SFH64 and the lens elements L.sub.2 and L.sub.3 consist of
polycarbonate and whose focal length is 90 mm, the following values
apply:
______________________________________ C(mm.sup.-1) di(mm) n
______________________________________ S.sub.1 0.003714 L.sub.1
102.00 1.712 S.sub.3 -0.007749 0.50 S.sub.4 0.006226 L.sub.2 19.00
1.573 S.sub.5 -0.003162 57.35 S.sub.6 -0.011750 L.sub.3 5.00 1.573
S.sub.7 -0.002857 ______________________________________
whilst the aspherical coefficients a.sub.2i of the surfaces
S.sub.1, S.sub.4 and S.sub.6 are equal to
______________________________________ S.sub.1 a.sub.2 = 0.185676
.times. 10.sup.-2 a.sub.4 = -0.249319 .times. 10.sup.-6 a.sub.6 =
-0.274689 .times. 10.sup.-9 a.sub.8 = 0.181459 .times. 10.sup.-12
a.sub.10 = -0.761700 .times. 10.sup.-16 a.sub.12 = 0.123104 .times.
10.sup.-19 S.sub.4 a.sub.2 = 0.311297 .times. 10.sup.-2 a.sub.4 =
0.187661 .times. 10.sup.-6 a.sub.6 = 0.297361 .times. 10.sup.-10
a.sub.8 =-0.811958 .times. 10.sup.-14 a.sub.10 = 0.241555 .times.
10.sup.-17 a.sub.12 = -0.232393 .times. 10.sup.-21 S.sub.6 a.sub.2
= -0.587496 .times. 10.sup.-2 a.sub.4 = -0.336531 .times. 10.sup.-5
a.sub.6 = 0.156573 .times. 10.sup.-8 a.sub.8 = - 0.690708 .times.
10.sup.-12 a.sub.10 = 0.203958 .times. 10.sup.-15 a.sub.12 =
-0.266676 .times. 10.sup.-19
______________________________________
Thanks to the novel design of this projection lens system the
distance between the display window and the entrance pupil of this
system can be increased by a factor of, for example 1.25 as
compared with a conventional projection lens system. This increase
in distance results in a reduction of the angles at which the chief
rays of the beams which originate from the display tube and are
accepted by the projection lens system are passed. For the purpose
of comparison the following Table shows the angles of acceptance
.alpha..sub.acc for different relative position P.sub.FP on the
display window for a conventional projection lens system PL.sub.C
and for a projection lens system PL.sub.I according to the
invention with a focal length of both 78 mm and 90 mm.
______________________________________ f = 78 mm f = 90 mm P.sub.FP
.alpha..sub.acc -PL.sub.C .alpha..sub.acc -PL.sub.I .alpha..sub.acc
-PL.sub.C .alpha..sub.acc -PL.sub.I
______________________________________ 0 0.0 0.0 0.0 0.0 0.25 10.7
10.2 8.8 6.7 0.50 20.5 20.3 17.1 13.8 0.75 29.2 27.9 24.9 20.2 1.00
37.6 31.2 32.8 24.0 ______________________________________
These values apply to a display window having a radius of curvature
.rho.=350 mm.
The angle of acceptance at a given position on the display window
is the angle between the normal of this position on the display
window and the chief ray of a beam starting from this position and
passing the projection lens system.
The effect of the improved projection lens system is greatest in
the corners of the display window for which the relative position
P.sub.FP =1.00 for which position also the field angle is defined.
The reduction of the angle of acceptance in the corner of the
display window is thus 6.4.degree. and 8.8.degree. for a projection
lens system having a focal length of 78 mm and 90 mm,
respectively.
In FIG. 11 the angle of acceptance .alpha..sub.acc,M in the corner
of the display window is plotted as a function of the focal length
f of a conventional projection lens system for both a display tube
having a flat display window, the solid-line curve 30, and a
display tube having a concave display window with a radius of
curvature r.sub.c =350 mm, the broken-line curve 31. In this Figure
is also indicated by the points 32 and 33 the maximum angles of
acceptance .alpha..sub.acc,M for the wavelengths 78 mm and 90 mm in
a projection system having a display tube with a concave display
window in which r.sub.c =350 mm and a projection lens system
according to the invention.
FIGS. 12 and 13 show the effect of the reduction of the angle of
acceptance on the gain in brightness G.sub.b which is obtained when
using an interference filter (with p=1.26) in the display tube. In
these Figures the relative position P.sub.PS on the projection
screen is plotted in the horizontal direction. The solid-line
curves 34 and 36 apply if a conventional projection lens system is
used and the broken-line curves 35 and 37 apply when using a
projection lens system according to the invention. As is shown FIG.
12 relates to a projection lens system having a focal length f=90
mm and FIG. 13 relates to a system with f=78 mm.
The transmission of the interference filter for the selected
central wavelength .lambda. is 50% for a direction at 37.degree. to
the normal on the display window.
When using a conventional projection lens system with f=90 mm, the
gain in brightness G.sub.b in the centre is 1.7 and it decreases to
1.0 in the corner of the projection screen. This decrease in
brightness is accompanied by a colour shading which cannot be shown
in FIG. 12. When using a projection lens system according to the
invention with a folded first lens element the decrease in the gain
in brightness is considerably less: from 1.7 in the centre to 1.5
in the corner of the projection screen. If a conventional
projection lens system with f=78 mm is used the gain in brightness
is 1.7 in the centre and 0.7 in the corner of the projection screen
as is shown in FIG. 13. Considerably better results are achieved
when using a projection lens system according to the invention with
a folded first lens element: the brightness at the edge of the
projection screen is then still 1.15 whilst the colour shading is
considerably reduced.
Although FIGS. 8, 9 and 10 show image projection systems with
display tubes having a concave display window, the invention is not
limited thereto. Also in projection systems in which the display
tubes have for example a flat display window a combination of the
special projection lens system and the interference filter in the
tube may increase the brightness on the projection screen and the
colour shading may be reduced at a relatively short focal length of
the projection lens system.
The invention may not only be used in a colour picture projection
system but also in a single colour projection lens system in order
to improve the distribution in brightness on the projection screen,
to increase the gain in brightness, to realize a better contrast
and to reduce chromatic aberrations.
* * * * *